INTRODUCTION

This technical report presents the results of statistical analyses of accident experience for interchange ramps and speed-change lanes. A background discussion to define the terminology used for interchange types and their components is presented below. The objectives and scope of this research and the organization of this report are then discussed.

Highway interchanges are systems of minor roadways designed to connect two or more major roadways. The major roadways connected at an interchange may consist of two fully access-controlled freeways, one freeway and one arterial highway, or two arterials. This research has focused exclusively on freeway-freeway and freeway-arterial interchanges. Arterial-arterial
interchanges were not considered.

Interchanges have many different possible configurations. The configuration chosen for the design of any particular interchange must be appropriate for the volumes of traffic making specific turning movements at the interchange, the alignments of the roadways being connected, the surrounding terrain, the adjacent development, and physical constraints such as existing rivers, railroads, and roadways. Figure 1 illustrates typical interchange configurations-from the simplest full-diamond interchange to complex, multi-level directional interchanges. Many
variations of each of these basic interchange configurations are possible.

The minor roadways that are provided within an interchange to allow traffic to move from one major roadway to another are known as ramps. Ramps come in various configurations appropriate to the design of the interchange in which they are located. Many ramp types are named after the interchange types in which they are most commonly used. Thus, the ramps of a diamond interchange are typically known as diamond ramps, and the loop ramps within a partial cloverleaf (parclo) interchange are typically known as parclo loop ramps. Figure 2 illustrates a number of typical
ramp configurations. Each of the ramps of the ramp configurations illustrated serves traffic exiting from a mainline freeway, but an analogous ramp configuration for traffic entering the mainline freeway also exists.

A ramp that leaves a mainline freeway facility is known as an off-ramp or exit ramp. A ramp that joins a mainline freeway facility is known as an on-ramp or entrance ramp. This distinction is important because vehicles typically travel along off-ramps at higher speeds than along
on-ramps, so that accidents are more likely to occur on off-ramps. Ramps that join mainline freeways at both ends serve as both off-ramps and on-ramps.

Figure 1. Typical Interchange Configurations

Figure 2. Typical Ramp Configurations

Ramps are connected to mainline freeways and, in some cases, to arterials by speed-change lanes that allow entering and exiting vehicles to speed up or slow down without
conflicting with through traffic. The speed-change lane for an off-ramp is known as a deceleration lane, while the speed-change lane for an on-ramp is known as an acceleration lane. Figure 3 illustrates the distinction between ramps and their adjacent speed-change lanes.

Most ramps connect directly to the adjacent mainline freeway by means of speed-change lanes. However, a few larger interchanges have intermediate roadways, known as collector/distributor roads or C/D roads, that connect the ramps and the speed-change lanes. Figure 1
illustrates a full cloverleaf interchange with C/D roads. Some basic descriptive statistics on the safety performance of C/D roads were assembled in this research, but no statistical modeling of accidents on C/D roads was performed.

Research Overview

The objective of this research study was to develop statistical models for
defining the relationships between traffic accidents and highway geometric elements, and traffic volumes for interchange ramps and speed-change lanes. It was hoped that these models could be used in predicting the effects on accidents of specific geometric design decisions at interchange ramps and speed-change lanes.

Several major technical tasks were performed during the research, including:

A review of previously published and unpublished literature concerning the relationship
between traffic accidents and interchange ramp and speed-change lane geometrics, as well
as between traffic accidents and highway geometric design features in general.

A review of existing policies, guidelines, standards, and practices for design of interchange ramps and speed-change lanes.

A review of existing highway agency files containing geometric design, traffic volume, and
accident data, including the data bases in the Federal Highway Administration (FHWA) Highway
Safety Information System (HSIS). As a result of these efforts, the data base from the Washington State Department of Transportation (DOT) was found to be best suited for the investigation of relationships between interchange ramp and speed-change lane geometrics and accidents, and was used for developing statistical models and testing statistical approaches in this research.

Figure 3. Illustration of Ramps and Adjacent Speed-Change Lanes

The development of statistical models for relationships between traffic accidents and geometrics. Alternative modeling approaches were investigated based on various assumptions about the distribution of accidents, including the Poisson and the negative binomial distributions. The goodness of fit of these various alternative models and the role of geometric design variables in these models were assessed. Statistical models were developed for various combinations of interchange elements.

The collection of additional data of geometric parameters for a sample of 200 ramps using
aerial photographs and other existing files of the Washington State Department of Transportation.
Additional statistical analyses incorporating these variables were conducted.

Scope and Organization of This Report

This report is organized into six main sections and four appendixes, including this introduction. Each section is briefly described below.

Section 2 provides a brief overview of the literature related to modeling traffic accidents. Advantages and disadvantages of the various statistical modeling approaches are discussed.

Section 3 provides a review of available accident and roadway files of State highway agencies, including the States in the HSIS. This section documents the reasons for selecting the Washington data base for this work.

Section 5 presents the results from various statistical models that were developed with Poisson and negative binomial regressions for various combinations of interchange elements. These results were derived from the Washington data base.